This invention relates to a differential amplifier circuit, and more particularly to a gain control circuit utilizing a double balance type cascade amplifier circuit and to a differential amplifier circuit in which the operating point of a gain control transistor and the DC potential of its output are stable at all times irrespective of the gain control operation.
A typical conventional differential amplifier circuit having a gain control functions is shown in FIG. 1. The gain control operation of the differential amplifier circuit is carried out by varying the DC emitter currents of transistors 1 and 2, i.e., the collector current of transistor 3 employed as a constant current source by means of a variable voltage source 8. In other words, an increase in the collector current of the transistor 3 decreases the emitter resistance of the transistors 1 and 2 in the input differential amplification stage. In such an ordinary differential amplifier circuit, in order to obtain a voltage gain control range of 20 dB for instance, it is necessary to change the emitter current by a factor of ten. Accordingly, in the case where the collector load of the transistors 1 and 2 is a resistance load, the resistance is, in general, several hundred ohms to 1 K.OMEGA.. Therefore, in general, the DC potential variation of the output reaches several volts or higher.
As was described above, in the conventional differential amplification circuit, the DC potential of the output is varied by the gain control operation. Therefore, in the case where the conventional differential amplification circuit is manufactured in the form of an integrated circuit, it is very difficult to design a circuit to be connected to the next stage. In addition, the amplitude-frequency characteristic of the output signal also varies in an amplification stage having a gain control function.